//--- Preparation ----

let wrapperNode = document.createElement('div');
let posibleGroupeKeys = ['fooo','barrrr','foo','bar'];

for (let i = 0; i < 129; i++) {
    let childNode = document.createElement('span');
    childNode.dataset.collectionGroup = posibleGroupeKeys[Math.floor(Math.random(0,posibleGroupeKeys.length)*posibleGroupeKeys.length)];
    wrapperNode.append(childNode);
}

let collections = wrapperNode.querySelectorAll('[data-collection-group]');

// ---- Block 1. ----

let groups = {};
collections.forEach((collection, index) => {
    let groupKey = collection.dataset.collectionGroup;

    if(!groups.hasOwnProperty(groupKey)){
        groups[groupKey] = [];
    }

    groups[groupKey].push(collection);
});

console.log(groups);


// ---- Block 2. ----

let collectionsArray = Array.from(collections);
let setOfGroupKeys = new Set(collectionsArray.map(element => element.getAttribute('data-collection-group')));

let groups = {};
setOfGroupKeys.forEach((groupKey) => {
 groups[groupKey] = collectionsArray.filter(elem => elem.dataset.collectionGroup === groupKey);
});


console.log(groups);

// ---- Block 3. ----

let groups = {};
collections.forEach((collection, index) => {
    let groupKey = collection.dataset.collectionGroup;

    if(groupKey && !(groupKey in groups)){
        groups[groupKey] = document.querySelectorAll('[data-accordion="true"][data-accordion-group="' + groupKey +'"]');
    }
});

console.log(groups);

// --- Block 4. ----

let groups = {};

groups = Array.from(collections).reduce((groups, node) => {
       let key = node.dataset.collectionGroup;
       if (!groups[key]) {
           groups[key] = [];
       }
       groups[key].push(node);
       return groups;
   }, {});
       
console.log(groups);

// --- Block 5. ----

let groups = {};
const collectionsLength = collections.length;

for (let i = 0; i < collectionsLength; i++) {
	let collection = collections[i];
    let groupKey = collection.dataset.collectionGroup;

    if(!groups.hasOwnProperty(groupKey)){
        groups[groupKey] = [];
    }

    groups[groupKey].push(collection);
}

console.log(groups);


// --- Block 6. ----

let groups = {};
const collectionsLength = collections.length;

for (const collection  of collections) {
    let groupKey = collection.dataset.collectionGroup;

    if(!groups.hasOwnProperty(groupKey)){
        groups[groupKey] = [];
    }

    groups[groupKey].push(collection);
}

   
console.log(groups);

/*
--- Result ----
{foo: Array(30), bar: Array(35), fooo: Array(30), barrrr: Array(34)}
bar: (35) [span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span]
barrrr: (34) [span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span]
foo: (30) [span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span]
fooo: (30) [span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span, span]
__proto__: Object
*/